1. Field of the Invention
The present invention relates to a printed wiring board and its manufacturing method.
2. Discussion of the Background
Multilayer buildup wiring boards are widely used as package substrates. Such a multilayer buildup wiring board is formed by building up an interlayer resin insulation layer with wiring one layer at a time on a core substrate where through holes are formed to be a signal line, a power-source line or a ground line. In an IC chip where high-frequency signals are transmitted, unless high-frequency properties are enhanced and impedance is lowered in the power-source line and ground line to be distributed to the package substrate, power supply through the power-source line falls behind and the earth potential level through the ground line may fluctuate to cause malfunctioning. In a package substrate to respond to high frequencies, frequency properties are improved by arranging multiple ground lines and power-source lines so as to achieve the same effects as with inductance in a parallel connection.
To connect wiring lines formed on both surfaces of a core substrate, a through hole is formed by using a drill to bore a penetrating hole in the core substrate or by using a laser to bore a penetrating hole, and then by forming a conductive layer inside. When through holes are formed, through holes with different diameters are formed by selecting the size of a drill or by properly selecting a drill or a laser (see Japanese Laid-Open Patent Publication 2001-168531). The entire contents of this publication are incorporated herein by reference.